Many recent studies have utilized IRS to secure the physical layer of wireless communications.
The authors in [1]}, [2]}, [3]} investigated an IRS-aided secure wireless system where a multi-antenna transmitter communicates with a single-antenna receiver in the presence of an Eve.
In [1]}, [2]}, [3]}, the SR was maximized by jointly optimizing the transmit beamforming and the IRS phase shifts. Specifically, the authors in [1]} proposed an alternating optimization (AO) algorithm to design two variables alternately, in which the optimal solution to the transmit beamforming was computed directly and the IRS phase shifts were optimized by using semidefinite relaxation (SDR) method. In [2]}, the transmit beamforming and the IRS phase shifts were also optimized in an alternating manner. In each iteration, the solution to the transmit beamforming was achieved in closed form, while a semi-closed form solution to the IRS phase shifts was obtained by adopting the Majorization-Minimization (MM) algorithm. The element-wise block coordinate descent (BCD) and AO-MM algorithms were developed for solving the problem efficiently in [3]}. Furthermore, the authors in [10]} investigated a minimum-SR maximization problem in a secure IRS-aided multiuser multiple-input single-output (MISO) broadcast system with multiple Eves. The problem was successfully solved by applying the path-following algorithm and AO technique in an iterative manner. Moreover, two suboptimal algorithms with closed-form solutions were developed to further reduce the computational complexity.
To further enhance the security, the AN is designed to disturb Eve.
In [11]}, AN was firstly considered in an IRS-aided secure communication system. Specifically, the achievable SR of the system was maximized by jointly optimizing the transmit beamforming with AN and IRS phase shifts. An efficient algorithm based on AO was developed to solve the problem sub-optimally. The authors in [12]} considered the resource allocation design in an IRS-aided multiuser MISO communication system. Aiming to maximize the system sum SR, the beamforming vectors, AN covariance matrix at the BS and phase shift matrix at the IRS were jointly optimized by applying AO, SDR and manifold optimization. The authors in [13]} investigated a secure IRS-aided multigroup multicast MISO communication system to minimize the transmit power subject to the SR constraints. First, an SDR-based AO algorithm was proposed and a high-quality solution was obtained. Then, to reduce the high computation complexity, a low-complexity AO algorithm based on second-order cone
programming (SOCP) was presented. Simulation result indicates that these designs can guarantee reliable
secure communication with the aid of IRS as shown in Figure. REF .
